Recognition by the platelet integrin alphaiibB3 of macromolecular ligands, particularly fibrinogen, is a key step in hemostasis. Activation of alphaiibB3 involves some of the largest conformational changes known in proteins, including leg extension and head opening. Several therapeutics that target alphaiibB3 are used during percutaneous interventions; however, they also induce the active conformation and can lead to thrombocytopenia. Understanding the structure of integrin alphaiibB3, how its conformation is regulated during platelet activation, and how it binds to macromolecular ligands and small molecule therapeutics, is key to a better understanding of vascular processes and developing improved therapeutics. Furthermore, alphaiibB3 is a common target of antibodies that are responsible for immune thrombocytopenia purpura (ITP), and understanding the molecular basis of ITP is important for improvements in therapy. Four aims address these needs. 1. Crystal structures and molecular dynamics examine the atomic pathway between the closed and open headpieces, including four intermediate states, and demonstrate opening by RGD and therapeutic RGD-mimetics of the headpiece. Structures of alphaiibB3 bound to RUC-2 advance second-generation integrin antagonists that do not induce opening. 2. How alphaiibB3 recognizes macromolecular ligands will be examined with complexes with one or more ligands including the fibrinogen yC module, fibronectin Fn3 modules 9 and 10, Del-1 EGF domains, and a disintegrin. The structures will show how the non-RGD portions of ligands contribute specificity and affinity. 3. We examine the biology of integrin activation by measuring monomeric affinity for the fibrinogen yC module on resting and activated platelets. The effects of function-perturbing or function-dependent Fab including LIBS Fab and ligand-mimetic Fab are correlated with the conformation of intact alphaiibB3 in complex with the Fab in EM. 4. We will examine the structural basis of ITP. Crystal structures of drug-dependent complexes between Fab and alphaiibB3 are examined in model systems using a drug with no known propensity for alphaiibB3 (quinine) or a specific antagonist (eptifibatide). A new concept is advanced that drugs can perturb the backbone conformation of antibody variable region loops. Complexes with intact alphaiibB3 in EM examine specificity of drug-dependent Fab from patients with ITP.